The suspension characteristics of agricultural tractor tyres

A method used to measure the radial suspension properties of agricultural tractor tyres is described. The stiffness and damping of a range of tyres have been measured. The effects on tyre stiffness and damping coefficient of rolling speed, inflation pressure, load, amplitude, frequency of vibration, driving torque, surface type, tyre size, ply rating, construction, wear and age are reported. A relationship is developed which enables the stiffness of a rolling tyre to be estimated from the tyre size, age and inflation pressure. It is shown that this is a more accurate estimate of rolling tyre stiffness than measurement of the stiffness of a stationary tyre. The measured tyre characteristics are used to predict the vibration of a single degree of freedom system and of a four wheel tractor. Significant improvements in accuracy are found when the results are compared with those obtained using stationary tyre characteristics. The frequencies of the natural pitch and vertical modes of vibration are usually predicted to within ± 10%. Predictions of rms acceleration levels are less accurate. Further improvements in modelling accuracy should be achieved by more accurate measurement and modelling of the suspension characteristics of the drive wheels in the longitudinal direction.